The invention relates to improvements in converters, and in particular to an improved apparatus for converting an input voltage, such as a direct current voltage (d.c.) or an alternating current (a.c) voltage, into an a.c. output voltage. It relates especially to converters for very high voltages employing semiconductor switching devices.
For high voltage transmission and distribution networks there is a need to convert voltages from one type or level to another. This may be to step down from a first voltage, for example 200 kVolt distribution voltage, to a lower level such as 66 kVolts for an urban network, or to convert a high voltage dc waveform to an alternating waveform to drive a load such as a motor. Such voltages are commonly found in both industrial and domestic power distribution systems as well as electrified railway systems.
Traditionally the conversion of such high voltages has been performed using a transformer protected by circuit breakers. The transformer comprises a primary having a winding of a first number of turn and a secondary having a winding with a second, different, number of turns. The output voltage will differ from the input voltage by the ratio of the turns on the windings. Transformers are bulky and inflexible in terms of their operation. It has therefore been proposed to employ semiconductor switching devices to construct a converter for very high voltages.
The use of semiconductor devices in converters for converting a supply voltage to an output voltage is well known in the art for relatively low voltages up to several hundreds of volts. By converter we mean an apparatus which is adapted to convert a first input voltage into an output at a second voltage. If the output voltage is at the same frequency as the input voltage then the converter can be used to replace a step up or step down transformer. Alternatively, the converter could be used to convert an alternating waveform (ac) to a steady state voltage at the output (dc) or vice versa.
Although the development of high power semiconductor switching devices is advancing rapidly, at present the maximum input voltage that can be handled by a solid state converter is quite limited due to the relatively low maximum switching voltage that can be safely handled by the individual switching devices used.
The applicant is aware of semiconductor switching devices that can handle peak voltages of 6.5 kVolts. To ensure that the instantaneous peak voltages that arise immediately after the devices are switched can be handled, then these devices limit the peak input voltage to around half of their peak rating, i.e. 3.6 kVolts. Whilst this is more than adequate for the production of converters for domestic voltage levels, this is far below the voltage handling capacity required for converting the high voltages used in transmission and distribution systems which can be up to 400 kVolts or more.
In order to increase the voltage capacity which can be safely handled, it has been proposed to provide a number of identical converters connected in series. Thus, two identical switching circuits could be provided in series to double the voltage handling capacity with the voltage being dropped equally across the two circuits. In practice this proves impractical for the production of converters for the high voltages used in distribution networks due to the extremely high parts count that would result and the need to synchronise the switching of the devices to a very high level of precision. If the circuits are not identical then the voltage may not be shared equally which could overload an individual circuit. This could lead to a catastrophic failure.